The Leather Industry
One of the oldest applications of industrially produced enzymes is in the processing of hides and skins for leather. Hides and skins contain proteins and fat in between the collagen fibres. Before the hides and skins can be tanned, these substances must be partially or totally removed. The proteins can be removed by proteases and the fat can be removed by lipases, as well as by tensides and solvents. Today, proteases are used mainly for soaking, bating and enzyme-assisted unhairing. The use of lipases to dissolve and remove fat is under rapid development and lipases are now an integrated part of leather processing in many parts of the world.
When preparing hides and skins for liming and unhairing, proper soaking of the rawstock is essential for obtaining a good quality leather.
For some raw materials, notably dried stock, satisfactory rehydration may be a difficult and time-consuming process. By degrading interfibrillary protein using proteolytic enzymes, water absorption is significantly facilitated and the soaking operation can be shortened.
Novozymes’s soaking protease works especially well at pH 8-9.
The conventional way to remove hair from bovine hides is to use lime and sodium sulphide in a hair-burning process. They dissolve the hair and open up the fibre structure.
Novozymes markets a protease that assists in the removal of hair. It is a unique protease because it is active at the very high pH of 12-13 found in the liming process.
Most importantly, enzyme-assisted unhairing results in a cleaner grain surface and improved area yield and softness. The use of this specific Novozymes protease also offers tanneries a number of options. For instance, the sulphide and lime requirements can be reduced by as much as 40% while maintaining the same liming time. Alternatively tanners can shorten the liming time by at least half without any loss of quality. Another possibility is to avoid the use of amines, which can be converted into carcinogenic compounds.
The hair-burning process is by far the most widespread. This specific Novozymes protease can also be used to advantage in the hair-saving process, which is more environmentally sound. The hair is not dissolved and can be filtered out from the liming float. In this way, it is possible to reduce COD by 50% and BOD by 35% in waste discharges.
Lipases are a type of enzyme that specifically degrades fat and so cannot damage the leather itself. Lipases hydrolyze not just the fat on the outside of the hides and skins, but also the fat inside the skin structure. Once most of the natural fat has been removed, subsequent chemical treatments such as tanning, re-tanning and dyeing have a better effect.
The Novozymes alkaline lipase has a pH range of 7-13 whereas the Novozymes acid lipase is recommended for acidic conditions, pH 3-7. Novozymes is the only company offering both an acid and an alkaline lipase.
The main advantages of using lipases are a more uniform colour and a cleaner appearance. Lipases also improve the production of hydrophobic (waterproof) leather, makers of leather for car upholstery have commented that ‘fogging’ is reduced. This is the term for the build-up of a film of chemicals on the inside of car windscreens.
Lipases represent a more environmentally sound method of removing fat. For bovine hides, lipases allow tensides to be replaced completely. For sheepskins, which contain up to 40% fat, the use of solvents is very common and these can also be replaced with lipases and surfactants. Solvents tend to dry out the skin and give it a pale colour.
If surfactants are used for sheepskins, they are usually not as effective and may be harmful to the environment. Stronger surfactants such as nonyl phenol ethoxylate have a better effect but they are more detrimental to the environment. When using Novozymes lipases, the original surfactant dosage can be reduced by at least 50% in the case of both sheepskins and pigskins. In addition, nonyl phenol ethoxylate can be substituted with more biodegradable surfactants.
To make the leather pliable, the raw material requires an enzyme treatment before tanning. This is called bating, whereby certain protein components are dissolved and can be washed away. The degree to which bating is applied is dependent on the desired character of the finished leather. Glove leather, for example, should be very soft and pliable and is subjected to strong bating, whereas leather for the soles of shoes is only lightly bated. Leather for shoe uppers falls between these two extremes.
Traditionally, dog or pigeon dung was used as a bating agent. Besides being a difficult process to control with unpredictable results, the dung did not exactly contribute to the creation of a pleasant working environment. The dung bates owed their softening effect to the action of proteases, and it was heralded as a great step forward in 1908 when the German chemist Otto Röhm patented the first standardized bate. This was based on pancreatic enzymes extracted from slaughtered animals and turned out to be a great success. Today, both bacterial proteases and trypsin (the traditional pancreatic protease) are used for bating.
Novozymes produces both types of enzyme. Novozymes also offers a new acid protease that works well at pH 2.5-7. It should be applied during the second bating of pickled or wetblue skin/hides.
Flexibility and synergy
In both the bating and degreasing process steps, Novozymes’s range offers the flexibility to add an enzyme suited to the conditions, whether acidic or alkaline.
In addition, two different enzymes may be added simultaneously. For example, Novozymes’s unhairing enzyme (NUE) and alkaline lipase make a very good combination for liming. In trials, these two enzymes were found to give synergy effects. The Novozymes unhairing enzyme opens up the hide, allowing the Novozymes alkaline lipase to remove fat, giving both improved unhairing and improved degreasing. Another good combination is the simultaneous use of a Novozymes bating enzyme and a Novozymes degreasing enzyme under acidic conditions. These are examples of new areas that Novozymes will be investigating in the future.
The standard Product Range for the Leather industry looks as follows. Most products are available in liquid as well as solid form, and in different concentrations. Please contact your local sales office for further details as well as with inquiries about special products not listed here.
Please note that all products listed are not necessarily available in all countries. Contact your local sales office for details.
An alkaline lipase for degreasing hides and skins.
A protease for acid bating of hides and skins.
An acid lipase for degreasing hides and skins.
A protease for rehydration of dried and salted hides.
NUE (Novo Unhairing Enzyme)
A protease for unhairing hides and skins.
PTN (Pancreatic Trypsin Novo)
A trypsin protease for neutral and alkaline bating of hides and skins.
A protease for neutral and alkaline bating of hides and skins.